1. Field of the Invention
The present invention relates to a distributed feedback semiconductor laser assembly (DFB laser assembly) including a DFB laser device and a heat sink onto which the DFB laser device is bonded. More specifically, the present invention relates to a DFB laser assembly having a high degree of wavelength stability, having a smaller range of variation in the emission wavelength with respect to a change in the operating current, and is thus suited to act as a light source for use in an optical communications system. The present invention also relates to a DFB laser module having such a DFB laser assembly.
2. Description of the Related Art
Typically a DFB laser device has in its resonant cavity a diffraction grating for periodically changing the real part and/or the imaginary part of the refractive index (complex refractive index) in the resonant cavity so that only laser light having a specific wavelength is fed-back to the resonant cavity for wavelength selectivity. Such a DFB laser device diffraction grating has a plurality of stripe layers disposed adjacent to the active layer. The stripe layers may be periodically arranged and may have a refractive index different from the refractive index of the adjacent layer in the resonant cavity. This configuration allows the DFB laser device to select an emission wavelength λDFB independently of the optical gain peak wavelength of the active layer. The emission wavelength λDFB has a relationship λDFB=2·neff·Λ between the space period Λ of the diffraction grating and the effective refractive index neff of the optical waveguide.
Having such a wavelength selectivity, a typical DFB laser device is considered to be a promising light source for use in a long-distance optical communications system. For example, such a DFB laser device incorporated in a butterfly type DFB laser module is commonly used as a light source in the long-distance optical communications system. In FIG. 1, which shows the configuration of a typical butterfly type DFB laser module in a sectional view thereof, the DFB laser module 60 is configured to have 14 pins and emits a laser beam via an optical fiber 68. The DFB laser module 60 includes a DFB laser device 52, a heat sink 54, and a thermistor 56. The DFB laser device 52 is bonded onto the heat sink 54 made of aluminum nitride (AlN), which is supported on a base 72 and acts to dissipate the heat energy generated in the DFB laser device 52. The thermistor 56 is also provided on the heat sink 54 and serves to measure the operating temperature of the DFB laser device 52.
The DFB laser module 60 is generally driven to generate a constant optical output power in an APC (Auto Power Control) mode when employed as a light source in an optical communications system. Accordingly, there is provided a monitor photodiode 71 for detecting the optical output power from the DFB laser device 52 and for delivering a control signal to generate a constant optical output power. The monitor photodiode 71 is disposed on the DFB laser side in a monitor photodiode block 70, which is provided on the base 72.
In a practical configuration, there is provided an external feedback circuit (not shown) for adjusting the current Im delivered from the monitor photodiode 71 at a constant value in order to control a drive current and thereby emit a laser beam at a constant optical output power.
There are interposed a first lens 62, an optical isolator 64, and a second lens 66 between the DFB laser device 52 and the optical fiber 68. The first lens 62 is a collimator lens which collimates the laser beam emitted from the DFB laser device 52 to allow the resulting beam to impinge upon the optical isolator 64. The optical isolator 64 is an optical component which prevents the reflected beam from entering through the optical fiber 68 and transmits only the laser beam incident from the first lens 62 toward the second lens 66. The second lens 66 collects the laser beam incident from the optical isolator 64 and then transmits the resulting beam toward the optical fiber 68.
There is provided a Peltier element 74 beneath the base 72 to cool down or heat up the base 72. The Peltier element 74 cools down or heats up the DFB laser device 52 via the base 72 and the heat sink 54 to maintain the operating temperature of the DFB laser device 52, which is measured by the thermistor 56, at a predetermined temperature. These components including the DFB laser device 52 are received in a package 76, where a laser beam emitted from the DFB laser device 52 advances through the first lens 62, the optical isolator 64, and the second lens 66 into the optical fiber 68, which is inserted through the side-wall of the package 76, and then propagates outwardly therethrough.
The DFB laser module 60 described above and incorporating therein the DFB laser device 52 is widely used as a signal light source in an optical communications system employing the wavelength division multiplexing scheme (or a WDM system) which enables a large-capacity transmission. To transmit wavelength multiplexed signals via an optical fiber that are superimposed on multiple laser beams having different wavelengths, the WDM system requires semiconductor laser devices employed as the light sources, typically DFB laser devices, to provide the strict accuracy of the emission wavelengths and thus a high degree of stability of the emission wavelengths for a long period of time.
The tolerance of the wavelength accuracy and emission wavelength stability depend on the scale of the WDM system and the wavelengths of the laser beams of the light sources. In general, a narrower step frequency or spacing between the adjacent frequencies of the DFB laser devices requires a smaller tolerance. Approximately, the wavelength accuracy and emission wavelength stability are desired to be less than ±0.2 nm for a 100 GHz spacing (at a wavelength interval of up to 0.8 nm) and about ±0.1 nm for a 50 GHz spacing (at a wavelength interval of up to 0.4 nm).
In general, long-term operation of a semiconductor laser device results in degradation of laser properties. Specifically, the lasing efficiency of the laser device may gradually degrade or the threshold current of the laser device may increase more or less after a long-term operation. In a DFB laser device driven in an automatic power control (APC) mode, a degradation in the lasing efficiency or an increase in the threshold current of the DFB laser device causes the feedback loop to increase the drive current in order to maintain the optical output power at a constant. However, such a change in the drive current causes the emission wavelength to vary significantly, thereby making it difficult to maintain the wavelength accuracy and stability required by the WDM system. Accordingly, as discovered by the present inventors, there is a need for a DFB laser device which provides a reduced range of variation in the emission wavelength thereof with respect to a change in the operating current, thereby providing a high degree of stability in the emission wavelength.